In order to ease the understanding of the descriptions and figures in the presentation of the present invention, an index of the used abbreviations is hereby given:    AAAS Ambient Audio Acoustic Signal    Ablock Acoustical channel's block    ADC (A/D) Analog to Digital Converter    ANC Active noise cancellation    ASYNC Acoustical SYNC    DAC (D/A) Digital to Analog Converter    DSP Digital Signal Processor    EA Electrical Audio Acoustic Signal    Eblock Electrical channel's block    ESYNC Electrical SYNC    FIR Finite Impulse Response    FxLMS Filter X LMS    GSM Generated Sequence Mark    GTT Generated Time Tag    Imic Inside Microphone    LMS Least Mean Square    QAAS Electrical Audio Acoustic Signal    QASYNC Quiet Acoustical SYNC    QEAAS Quiet Electrical Audio Acoustic Signal    QESYNC Quiet Electrical SYNC    RTC Real Time Clock    RTT Received Time Tag    Smic Singer Microphone    SNR Signal to Noise Ratio    SOF Start Of Frame    SYNC Synchronization Signal(s)    TEAAS Transmitted Electrical Audio Acoustic Signal    TESYNC Transmitted Electrical SYNC
Active noise cancellation (ANC) is a specific domain of acoustic signal processing that intends to cancel a noisy signal by generating its opposite acoustic signal (referred to as “antiphase signal”). The idea of utilizing antiphase signals has gained considerable interest starting from the 1980s, due to the development of digital signal processing means.
The present invention is a method and system for active reduction of predefined audio acoustic signals emitted from a predefine source or sources in a predefined area of choice.
In order to relate to prior art and to explain and describe the present invention, the terms used in the text are hereby defined:
The invention is aimed to reduce predefined audio acoustic noise in predefined area or areas, referred hereafter as “quiet zone(s)”, without reducing other ambient audio signals produced either inside or outside of the quiet zone(s), and without reducing any audio acoustic noise outside of the quiet zone(s). Inside the quiet zone(s) people experience substantially attenuation of the predefined acoustic noise, thus, able to converse, work, read or sleep without interference.
The “quiet zone(s)”, refers in the context of the present invention interchangeably to a public and/or private areas, indoors and/or outdoors.
The predefined audio acoustic noise referred to in the present text, originates from a specified noise source such as, but not limited to, a mechanical machine, human voice (e.g. snores, talk) or music from an audio amplifier via a loudspeaker.
The term “acoustic” as defined by the Merriam Webster dictionary (http://www.merriam-webster.com/dictionary/acoustic) is: a) “relating to the sense or organs of hearing, to sound, or to the science of sounds”; b) operated by or utilizing sound waves. The same dictionary defines the term “sound” in context of acoustics as: a) particular auditory impression; b) the sensation perceived by the sense of hearing; c) mechanical radiant energy that is transmitted by longitudinal pressure waves in a material medium (as air) and is the objective cause of hearing. The same dictionary defines “signal” in the context of a “sound signal” as “a sound that gives information about something or that tells someone to do something” and in the context of electronics as “a detectable physical quantity or impulse (as a voltage, current, or magnetic field strength) by which messages or information can be transmitted”. The term “audio” is defined by the Merriam Webster dictionary as: relating to the sound that is heard on a recording or broadcast. “Noise” in the context of sound in the present invention is defined as: a) a sound that lacks agreeable musical quality or is noticeably unpleasant; b) any sound that is undesired or interferes with one's hearing of something. The term “emit” is defined by the Merriam Webster dictionary as: “to send out”. The same dictionary defines the term “phase” as: a) “a particular appearance or state in a regularly recurring cycle of changes”; b) “a distinguishable part in a course, development, or cycle”. Thus “in-phase” means: “in a synchronized or correlated manner”, and “out of phase” means: a) “in an unsynchronized manner”; b) “not in correlation”. The term “antiphase” is logically derived and means: “in an opposite phase”, which means synced and correlated, as in in-phase, but opposed in course/direction”. Since acoustical wave is a movement of air whose direction alter back and forth rapidly, creating an antiphase acoustic wave means that the generated wave has the same direction-changes rate but in the opposite directions, and has same momentary amplitude.
The term MEL scale refers to a perceptual scale of pitches judged by listeners to be equal in distance from one another. In the context of this invention the MEL scale is used for calibrating the system.
FIR filter is an abbreviation for: Finite Impulse Response filter, common in digital signal processing systems, and is commonly used in the present invention
LMS is an abbreviation for: Least Mean Square algorithm, used to mimic a desired filter by finding the filter coefficients that relate to producing the least mean squares of the error signal (the difference between the desired and the actual signal). In the present invention it is deployed by the system's computers to evaluate the antiphase. Some variations of such a filter are common in the field. FxLMS is the filter use in the present invention.
In the context of the present invention additional terms are defined:
The term “system” in reference to the present invention comprises the components that operate together forming a unified whole and are illustrated in FIGS. 5 and 6. The structure and function of the components is explained in detail further on in the text.
The term “Audio Acoustic Signals” is any acoustical audio signal in the air, whose source may be natural and/or artificial. In the context of the present invention, it refers to the non-predefined audio acoustics that need not to be reduced.
The term “Ambient Audio Acoustic Signals” is referred to in the present text as: “AAAS”. Typically, AAAS can be generated by, but not limited to, a machine and/or human beings, and/or animals—as shown at FIG. 1; as a specific case example it can be music or other audio voices from audio amplifier, as shown at FIG. 2; and/or by other pre-defined acoustic noise source(s). In the present invention a single as well as a plurality of predefined AAAS directed towards (a) quiet zone(s) is/are referred to a as referred to interchangeably as “targeted AAAS” and “predefined acoustic noise”. In the current invention, the predefined AAAS is/are the signal(s) to be reduced at the quiet zone(s) while the Audio Acoustic Signals are not reduced.
The term “acoustical distortion” means in context of the present text: the infidelity, or the misrepresentation of an acoustic signal at a specific location, in regards to its source, by means of its acoustical parameters such as: frequencies components, momentary amplitude, replications, reverberations, and delay.
The term “antiphase AAAS” in the context of the present text describes the precise momentary amplitude of the signal that opposes (negates) the original predefined AAAS as it actually arrives to the quiet zone, i.e. after it was acoustically distorted due physical factors. More specifically, the antiphase AAAS acoustical air pressure generated by the system at the quite zone is the negative acoustical air pressure originated by the predefined AAAS source, as it distortedly arrives to the quite zone. The present invention deals dynamically with this distortion.
Active canceling of predefined AAAS in a quiet zone is achieved by the acoustical merging of a targeted AAAS with antiphase AAAS. The canceling of the predefined AAAS by the antiphase AAAS is referred to interchangeably as “destructive interference”.
In the present text the terms: “earphones” and/or “headphones” are interchangeably referred to as “Quieting Loudspeakers”.
In the present invention antiphase AAAS is generated in the quiet zone(s) and broadcasted to the air synchronously and precisely in correlation with the predefined AAAS. This is done by using a unique synchronization signal, abbreviated as: SYNC.
Relating to prior art, presently there are commercial systems that generate antiphase signals in response to AAAS. These systems typically, but not exclusively, relate to headphones that include an internal microphone and an external microphone. The external microphone receives the AAAS from the surroundings and forwards the signal to a DSP (Digital Signal Processor) that produces appropriate antiphase AAAS that are broadcasted by a membrane inside the headphones. The internal microphone receives AAAS from within the confined space of the headphones and transmits it to the processing system as feedback to control and eliminate the residuals AAAS. Typically, headphones also provide an acoustic physical-barrier between the external AAAS and the internal space in the headphones. Also commercially available are systems that comprise an array of microphones and loudspeakers that generate antiphase AAAS in a relatively large area exposed to AAAS, thus, eliminating the AAAS penetrating a specific zone by creating a sound canceling barrier.
The advantage of the quieting Active Noise Cancellation (ANC) headphones” is the ability to control the antiphase signals to provide good attenuation of the received AAAS.
The disadvantage of “quieting ANC headphones” is the disconnection of the user from the surroundings. The wearer cannot have a conversation or listen to Audio Acoustic Signals while wearing the headphones. In addition, the ANC headphones mostly attenuate the lower frequencies of the audio spectrum, while the higher frequencies are less attenuated.
The quieting ANC headphones are mostly effective when AAAS is monotonous (e.g. airplane noise). When intending to achieve quiet with non-wearable equipment a complex array of microphones and loudspeakers is required for the sharp distinguishing, or barrier, between the noisy and quiet zones. The disadvantages are the high costs and large construction requirements.
In locations exposed to monotonous and repetitive AAAS, such as in, but not limited to, airplanes, refrigeration-rooms and computer-centers, the AAAS are typically characterized by limited frequency band in the range of up to about 7 KHz. Since in these cases the AAAS is frequency-limited, it becomes relatively easy to predict it, thus, to generate and broadcast appropriate antiphase AAAS in a designated quiet zone. This broadcast is done via loudspeakers, or, in specially designated headphones. Systems for the elimination of monotonous and repetitive AAAS or in low frequencies AAAS are available on the market.
Reference is presently made to AAAS in the context of the present invention:
Since AAAS (typically a combination of music and/or vocal acoustic signals) are difficult to predict, as they are non-stationary (i.e. typically not repetitive and they are typically cover large spectrum of human hearing ability, including high frequencies signals), it is not a simple task to generate a fully effective antiphase AAAS to achieve desired quiet zones. Typically, systems for creating quiet zones are limited to headphones. If a quiet zone is desired in a space significantly larger than the limited volume of the ear space (e.g. around a table, or at least around one's head), multi directional loudspeakers emitting the antiphase AAAS are required.
In order to substantially reduce AAAS whose source is located more than a few centimeters from a quiet-zone, the distortion of the AAAS due to its travel from the source to the quiet zone (the time-elapse for sound waves to spread through the air) has to be taken into account. The calculation to cancel the AAAS has so to fully adapt to the momentary amplitude, reverberations, frequency-response, and timing while broadcasting the antiphase AAAS. The present invention solves this problem and offers dynamic adaptation to environment's parameters, by on-line calculating the channel's behavior and response to a known stationary signal which is the SYNC.
Since the SYNC propagation in air has the same path as the undesired noise, it is possible to dynamically evaluate the distortion of the acoustical path, and the antiphase signal that is generated using SYNC distortion calculation.
In order to overcome the difficulties in precise correlation between the AAAS and the antiphase AAAS, various systems and methods have been disclosed, none of which have been fully successful in creating a distinct “quiet zone” in a distance of more than a few tens of centimeters from the source of the AAAS.
AAAS can be effectively eliminated at a distance of only a few tens of centimeters from its source, in a spatial volume having a narrow conical shaped configuration, originating from the AAAS source.
AAAS propagates in the environment in irregular patterns, not necessarily in concentric or parallel patterns, thus, according to prior art disclosed in U.S. Pat. No. 7,317,801 by Amir Nehemia, in order to reduce AAAS emitted by a single or several sources in a specific location, a single loudspeaker that emits antiphase acoustic signals is insufficient. Typically, the effective cancelation of incoming AAAS at a quiet zone requires the broadcasting of several well synchronized and direction-aimed antiphase acoustic signals to create an “audio acoustic protection wall”.
To overcome the necessity of an “audio acoustic protection wall” which in many cases is ineffective or/and requires expensive audio acoustic systems, U.S. Pat. No. 7,317,801 discloses an active AAAS reduction system that directly transmits an antiphase AAAS in the direction of the desired quiet zone from the original AAAS source. The effect of Amir's AAAS reduction system depends on the precise aiming of the transmitted antiphase AAAS at the targeted quiet zone. The further away the quiet zone is from the source of the AAAS, the less effective is the aimed antiphase AAAS. The quiet zone has to be within the volume of the conical spatial configuration of the acoustic signal emitted from the antiphase AAAS source.
Amir's system comprises an input transducer and an output actuator that are physically located next to each other in the same location. In one embodiment, the input transducer and the output actuator are a hybrid represented by a single element. The active noise reduction system is located as close as possible to the noise source and functions to generate an “anti-noise” (similar to antiphase) cancellation sound wave with minimum delay and opposite phase with respect to the noise source. In order to overcome sound-delay and echo-effects, a transducer in an off-field location from the source of the AAAS receives and transmits the input to a non-linearity correction circuit, a delayed cancellation circuit and a variable gain amplifier. The acoustic waves of the canceled noise (the noise plus the anti-noise cancelation which are emitted to the surrounding) are aimed at or towards a specific AAAS source location, creating a “quiet zone” within the noisy area. If an enlargement of the quiet zone is required, several combined input transducer and an output actuator need to be utilized.
Most prior art systems refer to the reduction of the entire surrounding noise, without distinguishing between the environmental acoustic audio signals. The method and system of the present invention reduces noise selectively.
An example of a disguisable noise reduction system is disclosed in US 20130262101 (Sriram) in which an active AAAS reduction system with remote noise detector is closely located to the noise source and transmits the AAAS signals to a primary device where they are used for generating antiphase acoustic signals, thus reducing the noise. Thereby, acoustic signal enhancement in the quiet zone can be achieved by directly transmitting antiphase AAAS in the direction of the desired quiet zone from the original AAAS source.
The method and system of the present invention reduces noise selectively. I.e. only predefined audio acoustic noise is attenuated while other (desired) ambient acoustic audio signals are maintained. Such signals may be, not limited to, un-amplified speaking sounds, surrounding voices, surrounding conversations, etc. The method is based on adding synchronization signals over the predefined signal, both electrically and acoustically, thus distinguish the predefined signal from others.